The common image of an insect often includes the large, multifaceted compound eyes of a fly or a butterfly, but the larval stage of moths and butterflies presents a surprising difference. While the adult insect possesses complex visual organs, the caterpillar has a completely distinct and numerous set of eyes. Instead of two main visual structures, a caterpillar typically has a collection of 12 simple eyes arranged on its head. This unexpected number reflects a form of vision perfectly suited to the caterpillar’s lifestyle.
The Stemmata Count and Arrangement
The simple eyes of a caterpillar are technically known as stemmata, or sometimes lateral ocelli. The vast majority of species possess six of these structures grouped closely together on each side of their head capsule, resulting in a total count of 12 visual organs. These stemmata appear as small, dark, bead-like spots situated low on the sides of the head near the mouthparts.
The arrangement is not a single, continuous visual field but rather six individual sensors clustered together. Although six is the typical number per side, some species may have five or seven stemmata on each side, though 12 remains the standard total. Each stemma has a single lens made of cuticle covering a small number of photoreceptor cells, which is a stark contrast to the thousands of units in a compound eye. This unique grouping and structure define the caterpillar’s visual system, which is optimized for function.
Visual Limitations and Purpose
Unlike the compound eyes of the adult butterfly, the stemmata do not produce a sharp, composite picture of the environment. The limited number of photoreceptor cells in each stemma means the caterpillar’s world is perceived as a coarse, low-resolution experience. These simple eyes function mainly to detect changes in light intensity and distinguish between light and dark.
This limited vision is entirely sufficient for the caterpillar’s primary purpose of eating and seeking shelter. The ability to sense light helps the larva orient itself, often guiding it toward the brighter areas of a plant where fresh foliage is available. Furthermore, the stemmata are capable of detecting movement, allowing the caterpillar to perceive the shadow or approach of a potential predator. In some social species, specialized stemmata can even detect the polarization of light, which helps the larvae navigate and maintain a straight course during group processions.